The rate of carbon dioxide emissions (CO2) from new cars and light trucks in the United States dipped for the first time in two decades, but their overall contribution to global warming has continued to grow steadily since 1990, according to a new report released on Aug. 30 by Environmental Defense.

The report, "Automaker's Corporate Carbon Burdens: Update for 1990-2005," shows that while the average CO2 emissions rate from new vehicles fell 3 percent from 2004 to 2005, it remained up a net 1.5 percent since 1990.

Leading the pack in emissions reductions were Toyota and BMW, which trimmed their CO2 emissions rates while gaining market share. Toyota's 3 percent drop in fleet-wide emissions rate from 1990 to 2005 was achieved in part by the Prius hybrid, but more so by incremental efficiency improvements to the Corolla. BMW's fleet average CO2 emissions dropped more than 12 percent due to efficiency improvement across its lineup along with the introduction of the Mini Cooper. Trailing behind were America's Big Three -- GM, Ford, and DaimlerChrysler -- which all saw a net worsening of their fleet-average CO2 emissions.

"The ability of Toyota and BMW to gain market share while cutting emissions is a clear example of innovative design paying off for the bottom line and the environment," said John DeCicco, senior fellow, automotive strategies at Environmental Defense.

The report also tracks automakers' overall "carbon burden," a useful metric for measuring progress in reducing cars' impact on global warming. Carbon burden reflects two key factors that contribute to CO2 emissions: the efficiency of vehicles and the carbon intensity of the fuel they run on, as well as new vehicle sales. "A complete climate policy solution for the auto sector is one that results in progressively lower carbon burdens," DeCicco said. "This can only be accomplished through climate-friendly innovations in both vehicles and fuels."

Highlights from the report regarding the CO2 emissions performance of the six largest automobile manufacturers (the Big Six) between 1990 and 2005 include:

General Motors' fleet average CO2 emissions rate was up 3 percent while its market share declined by 10 points; while GM's carbon burden fell 6.5 percent, it remained the largest overall.

Ford's CO2 emissions rate went up 4.7 percent while its carbon burden dropped nearly six percent due to a seven point loss in market share.